It is known in the art relating to internal combustion engines to provide one or more combustion chambers having inlet ports with openings for intermittently admitting to the chambers charges for combustion therein. In piston engines, the inlet ports commonly include inlet openings controlled by poppet valves and fed through inlet passages. These inlet passages may be defined as comprising the outer or lead-in portions of the cylinder intake ports and the connected runners of associated intake manifolds.
In connection with these arrangements, it has been previously recognized that a manifold or induction system designed for free breathing and good high speed performance is generally deficient in providing high engine torque at low speeds or efficient operation at part throttle and idle. A known method for improving this condition is to increase the flow velocity and inertia of the incoming air or air-fuel charge. This is sometimes attempted by providing dual manifold runners for each engine intake port, one runner having a relatively large cross section for free breathing at high speeds and loads and the other runner having a much smaller cross section to increase the flow velocity and inertia of the charge delivered to the intake ports. Another proposed solution is to provide the manifold runners with flexible walls to allow variation of their cross sections under differing operating conditions.
It is thought that the increased inertia resulting from utilizing intake passages or runners of smaller cross-sectional area during operation with reduced mass flow of the intake charges can result in increased cylinder turbulence and thereby improve combustion efficiency. The higher inertia may also improve volumeric efficiency by overcoming the back flow of a fresh intake charge when the piston begins its compression stroke and the intake valve is still open, a result which occurs predominatly at low engine speeds.